1. Field of the Invention
This invention relates generally to a novel flexible magnetic disc, and more particularly to a flexible magnetic disc of the type which has a hub structure for mounting the magnetic disc in position on a drive shaft of a magnetic disc drive unit.
2. Description of the Prior Art
Flexible magnetic discs such as floppy discs frequently have a hub structure including a center core secured to a central portion of a flexible magnetic sheet having a circular outer periphery. Such a floppy disc is rotatably carried by an outer rectangular case with the center core of the floppy disc exposed through the exterior of the case. When the floppy disc is to be used, the center core is mounted on a disc rotating shaft provided on a magnetic disc drive unit.
Various types of center cores are known in the prior art. One such center core for a 2-inch floppy disc has a pair of positioning surfaces on an inner periphery of an engaging hole formed at the center of the disc and includes a metal spring plate for resiliently pressing the positioning surfaces of the center core against a disc driving shaft of a magnetic disc drive unit. When the magnetic disc is to be mounted in position on the disc driving shaft, the shaft is inserted into the engaging hole of the magnetic disc whereupon the positioning surfaces of the center core of the magnetic disc and an inner face of the spring plate opposing the positioning surfaces contact an outer periphery of the disc driving shaft to resiliently align the middle of the center core with the axis of the disc driving shaft.
A flexible magnetic disc having a center core as described above is disclosed, for example, in U.S. Pat. No. 4,630,156 assigned to the assignee of the present application and is shown schematically in FIGS. 10 and 11 of the instant application.
Referring to FIGS. 10 and 11, a flexible magnetic disc (a) includes a flexible magnetic sheet (b) and a center core (c) secured to a central portion of the magnetic sheet (b). The center core (c) has an engaging hole (d) formed at a central portion thereof, the engaging hole (d) having a pair of positioning surfaces (e) formed on an inner periphery thereof. The positioning surfaces (e) lie on two equal sides of a substantially isosceles or equilateral triangle as seen in the plan view of FIG. 10, while a spring plate (f) is positioned along the remaining side of the triangle. The spring plate (f) is supported at the longitudinally opposing ends thereof in a pair of slits (g) formed in the inner peripheral portion of the engaging hole (d) of the center core (c) such that the spring plate (f) may be deformed laterally, that is, in a direction toward or away from the positioning surfaces (e).
The magnetic disc (a) is rotatably held in a cassette case (not shown), and when the cassette case is positioned in a cassette receiving section of a magnetic disc drive unit, a disc driving shaft (h) provided in the cassette receiving section is inserted into the engaging hole (d) of the center core (c) of the magnetic disc (a). In this instance, the positioning surfaces (e) of the center core (c) and a lower end face (i) of the spring plate (f) are contacted with and then slide downwardly along an inclined face (k) of a top portion (j) of a truncated conical surface of the disc driving shaft (h), and thereupon, the spring plate (f) is resiliently deformed away from the positioning surfaces (e) by the inclined face (k) of the disc driving shaft (h) to permit the magnetic disc (a) to be fitted in position onto the disc driving shaft (h). Meanwhile, the positioning surfaces (e) of the center core (c) are resiliently pressed against an outer periphery of the disc driving shaft (h) under the biasing force of the spring plate (f), thereby aligning the center of the magnetic disc (a) with the axis of the disc driving shaft (h). When the magnetic disc (a) is mounted in position on the disc driving shaft (h), the center core (c) thereof is attracted to a magnet (not shown) supported on a turntable (not shown) secured to the disc driving shaft (h) so that the magnetic disc is securely held in position on the turntable.
Such a conventional center core (c) has the drawback that the spring plate (f) may be caught by the top portion (j) of the disc driving shaft (h) and, thus, might not be loaded in the desired position. More specifically, when the disc driving shaft (h) is inserted into the engaging hole (d) of the center core (c), the lower end (i) of the spring plate (f) normally contacts the inclined face (k) of the top portion (j) of the disc driving shaft (h). However, in some cases the lower end (i) of the spring plate (f) first comes in contact with a top face (m) of the top portion (j) of the disc driving shaft (h) and then the center core (c) is displaced laterally in the cassette case after which the lower end (i) of the spring plate (f) contacts the inclined face (k) at the top portion (j) of the disc driving shaft (h). In the latter case, however, an edge of the lower end (i) of the spring plate (f) sometimes cuts into the inclined face (k) or the top face (m) of the disc driving shaft (h). This may prevent further downward movement of the center core (c). Consequently, the disc driving shaft (h) cannot then be inserted into the engaging hole (d) of the center core (c) of the magnetic disc (a).
Moreover, if the lower end (i) of the spring plate (f) cuts into the top portion (j) of the disc driving shaft (h), then the top portion (j) will be damaged. Consequently, even if the lower end (i) of the spring plate (f) is then released from the top portion (j) of the disc driving shaft (h) to permit positioning of the magnetic disc (a), when the same or another magnetic disc (a) is to be subsequently mounted onto the disc driving shaft (h), the probability is increased significantly that the spring plate (f) will be caught by the damaged portion of the disc driving shaft (h) and cannot then be loaded into position.